The present invention relates to an image forming apparatus that exposes and records an image by controlling the light emission amount of each light emission element such as an organic electroluminescence element (hereinafter referred to as the “organic EL element”) using an image input signal, and an image forming method.
A color print system for printing out an image of a photograph or the like records the image as a latent image by exposing a photosensitive material with modulated light and then prints out the image by processing and developing the latent image.
Nowadays, various techniques are proposed with which in the color print system, organic EL elements are used as a light source for exposing the photosensitive material.
Each organic EL element includes a cathode electrode forming an electron injection electrode, an anode electrode forming a hole injection electrode, and an organic layer that is a thin film arranged so as to be sandwiched between these electrodes and containing a fluorescent organic compound. With this construction, by causing a current (drive current) to flow between these electrodes, electrons are injected into the cathode and holes are injected into the anode. Then, recombination energy is generated through the recombination between the holes and the electrons and the organic layer is excited by this energy. Following this, at the time of transition from an excited state to a ground state, the organic layer emits fluorescence, thereby performing light emission.
The organic EL element having such a construction is influenced by moisture and oxygen around it and there easily occurs a dark spot that is a portion of the light emission region of the organic EL element where no light emission is performed. In view of this problem, a dry process is used for the production of the organic EL element. Also, the organic EL element is produced and constructed using various schemes for excluding the moisture and oxygen, such as the refinement of the organic material in the organic EL element, the adjustment of the vacuum degree at the time of film formation, and the sealing of the organic EL element. At this stage, however, it is extremely difficult to produce the organic EL element so as to have a sufficient light emission characteristic by suppressing the occurrence of the dark spot.
In addition, aside from the occurrence of the dark spot, the organic EL element has other problems such as a brightness degradation problem in that the brightness of the organic EL element is lowered along with the use. Therefore, it is also difficult to realize an apparatus that outputs a high quality image while achieving a long life span.
A printer apparatus using the organic EL element as a light source is proposed in JP 2000-305191 A, although a practical color print system is not yet provided at present.
Also, JP 2000-52592 A discloses a print head, which exposes a photosensitive material using the light of the organic EL element, and a drive method therefor. This print head is made in order to solve the problems such as the dark spot problem that becomes a cause of streak unevenness of a formed image and the life span problem (brightness degradation problem) in that the brightness of the organic EL element is lowered along with the use. This head using the organic EL element forms a color image by overlay-writing three images in the three primary colors on a single photosensitive plane (multiplex-exposure). It is said in this patent document that with this print head, it becomes possible to extremely reduce the influence of the conventional dark spot problem.
By the way, in the case of an exposure recording apparatus that exposes and records an image on a photosensitive material using light emission elements such as a laser light source, a so-called pulse-width modulation system is used. When an image input signal of an image is supplied, the density of the image that should be formed on the photosensitive material is determined from this image input signal. The exposure light amount and the light emission amount of each light emission element are determined so that the density is realized, a pulse control signal is generated in which a pulse width has been controlled so that the light emission amount is realized, and the light emission element performs the exposure and recording.
When the organic EL element described above is used as the light emission element in the exposure recording apparatus having such a construction, from the viewpoint of ease of control, a method is conceived with which an image is exposed and recorded on a photosensitive material using the pulse width modulation system. In this case, the image is recorded by causing the organic EL element to emit light having a light emission amount corresponding to an exposure light amount required by the photosensitive material. Also, in this case, as can be understood from the light emission mechanism described above, the organic EL element is caused to perform the light emission through the control of a drive current, that is, through the linear control of the signal level of the drive current where the light emission amount of the organic EL element has been quantized.
It is conceived, however, that in such control of the light emission amount of the organic EL element, the gradation reproducibility of the image may not become appropriate due to the characteristics of the photosensitive material and extreme differences occur between the levels of density gradation.
It is considered that with the print head disclosed in JP 2000-52592 A, it becomes possible to reduce the influences of noises, such as the dark spot, by forming an image through multiple-exposure. In this print head, however, an image is formed through density modulation with multiple-exposure by organic EL elements different in each pixel position of the image to be recorded, so that there is a problem in that it is not necessarily possible to appropriately reproduce the gradation of an image having continuous gradation levels of half tones. That is, when the multiple-exposure is performed in image formation in the manner described above, this results in a situation where the flexibility of an image forming method is limited, which leads to a problem in that the gradation reproducibility of an image is limited.